Add test cases for transferring type specifiers

Turns out we do support `transferring` already.

rdar://123876615

Author: ahoppen

Tests/SwiftParserTest/DeclarationTests.swift

@@ -3156,4 +3156,19 @@ final class DeclarationTests: ParserTestCase {
       XCTAssertEqual(decl.description, input, line: line)
     }
   }
+
+  func testTransferringTypeSpecifier() {
+    assertParse(
+      "func testVarDeclTupleElt() -> (transferring String, String) {}",
+      experimentalFeatures: .transferringArgsAndResults
+    )
+    assertParse(
+      "func testVarDeclTuple2(_ x: (transferring String)) {}",
+      experimentalFeatures: .transferringArgsAndResults
+    )
+    assertParse(
+      "func testVarDeclTuple2(_ x: (transferring String, String)) {}",
+      experimentalFeatures: .transferringArgsAndResults
+    )
+  }
 }